Here. In short,
the patent that Immersion sued Sony over claims priority to a patent from November 30, 1995, and patents are good for 20 years from the priority date.
The patent covers stuff like rumble in the DualShock and force feedback in steering wheels. Basically, the terms of the original settlement said that Sony had to negotiate a new license agreement for every new version of the PlayStation (PS1, PS2, etc.) and they were only allowed to sublicense the tech to other companies specifically making controllers for the appropriate PlayStation.
So if Sony wants to make DS4s work on a PC, or if Logitech wanna make their PS3 wheels compatible with PS4, then new deals need to be negotiated with Immersion. But come December, they'll be able to do whatever they want, and Immersion won't be able to say anything about it. (Assuming they didn't sign contracts that extend past the expiry of the patent.)